As is known in the art, in many applications it is necessary to remove heat from, for example, microelectronic components such as semiconductor integrated circuit chips. One heat removal or, more generally, thermal management technique, includes the use of a thermal interface structure disposed between the integrated circuit chip and a heat sink,
As is also known in the art, the trend of increasing power density in microelectronic components has accelerated the need for improved thermal management techniques. With heat fluxes ranging from 0.1-3 kW/cm2 being expected in next generation silicon and wide band gap semiconductor electronics, reducing thermal resistances across interfaces has become crucial in reducing device operational temperature and ensuring reliability. This research challenge has led to the development of new thermal interface materials along with advanced heat spreaders (e.g., diamond, AlN, etc.) in order to effectively manage heat dissipation requirements. At present, the highest performing thermal interface materials commercially used are in the form of solder die attach films. These films have thermal conductivities which range from 20-86 W/mK. However, they are susceptible to thermal fatigue and aging, are not always easily reworkable, and may transfer large stresses to the die which limits die thinning concepts.
Multiple approaches and various technologies have been evaluated to enhance thermal transport across interfaces in past research efforts. Although extensive experimental work has been conducted on different epoxy filler materials (i.e. silver, diamond, Carbon Nano-Tube (CNT), carbon fiber, see U.S. Pat. No. 7,316,661, issued Jan. 8, 2008), as well as the size, shape, and volume concentration of filler, there has been very little effort towards the creation of alternative approaches to filled epoxies. Most prior experimental work focused on improving bulk thermal conductance and was guided by percolation theory. The current thrust across the industry is to understand interfacial resistance at joints, and the reason for the dramatic reduction in performance that these interfaces pose. Improved understanding of thermal energy transport at nanometer scales in recent years has enabled a broad range of technological advances.
More particularly, extensive experimental work has been conducted on particle filled materials (e.g. silver, diamond, CNT and carbon fiber), low-temp solders and vertically aligned nanostructures. Most prior experimental work was focused on improving bulk thermal conductance (see E. E. Marotta and L. S. Fletcher, “Thermal contact conductance of selected polymeric materials,” J. Thermophys. Heat Transf., vol. 10, no. 2, pp. 334-342, 1996) and was driven by percolation theory or involved direct growth of CNTs/Carbon Nano Fibers (CNF) on device substrates. The current thrust across the industry is to understand interfacial resistance at joints, and the reason for the dramatic reduction in performance that these interfaces pose (see E. E. Marotta and L. S. Fletcher, “Thermal contact conductance of selected polymeric materials,” J. Thermophys. Heat Transf., vol. 10, no. 2, pp. 334-342, 1996).
Vertically aligned CNTs are well documented as providing exceptional on-axis thermal conductivity (see D. E. Angelescu, M. C. Cross, and M. L. Roukes, “Heat Transport in Mesoscopic Systems,” Superlatices and Microstructures, vol. 23, p. 673, 1998); however, both multi-(MW) and single-walled (SW) CNTs have yet to reach their potential in interface applications. While possessing greater theoretical conductivity, SWCNTs are problematic in vertically aligned array interface applications as their diameters (1-2 nm) (see J. M. Ziman, Electrons and phonons: the theory of transport phenomena in solids. Oxford: Clarendon Press, 1962) are smaller than the dominant phonon wavelength in most growth substrates (˜5 nm) (see M. S. Dresselhaus, G. Dresselhaus, and P. C. Eklund, Science of Fullerenes and Carbon Nano-tubes. SanDiego, Calif.: Academic Press, 1996), resulting in scale-mismatch induced reflection that increases interfacial resistance. Some MWCNT approaches have utilized polymer (see R. Prasher, “Thermal interface materials: Historical perspective, status, and future directions,” Proceedings of the Ieee, vol. 94, pp. 1571-1586, August 2006) or metal fillers (Chuang, H. F. et al., “Improvement of Thermal Contact Resistance by Carbon Nanotubes and Nanofibers,” Journal of Nanoscience and Nanotechnology, Vol 4, no. 8, pp. 964-967, 2004) to provide mechanical stability and adhesion. Elastic medium interference with phonon dispersion and lack of compliance to micro-scale surface roughness present in real substrates has limited the efficacy of these approaches. Recently, double sided CNT-foil gaskets synthesized at Purdue have exhibited promise as interface materials for micro-rough surfaces, exhibiting resistances of ˜10 mm2K/W (see B. A. Cola, X. Xu, T. S. Fisher, Applied Physics Letters 2007, 90, 093513) under moderate pressures.
In related work, CNT free-end contact resistance has been shown to comprise ˜90% of the overall resistance in Cu-CNT-Si interfaces (see J. Xu and T. S. Fisher, “Enhancement of thermal interface materials with carbon nanotube arrays,” International Journal of Heat and Mass Transfer, vol. 49, pp. 1658-1666, May 2006). Metallic bonding of CNT free-ends using Indium solders (see T. Tong, A. Majumdar, Y. Zhao, A. Kashani, “Indium Assisted Multiwalled Carbon Nanotube Array Thermal Interface Materials” 2006 IEEE proceedings) has been shown to reduce thermal resistance an order of magnitude vs. a dry interface.
To be an effective alternative to the presently employed thermal epoxies an alternative Thermal Interface Materials (TIM) solution for high-power electronic devices must not require modification to the device or heat sink, be compatible with device metallization schemes, allow factory rework, allow for some degree of heat sink roughness and absorb coefficient of thermal expansion (CTE) mismatch between the device and heat sink so not to generate stress.